Patent Document 1 discloses an engine which combusts air-fuel mixture in a combustion chamber by compression autoignition in a partial load region. This engine promotes autoignition of air-fuel mixture by leaving hot burned gas in the combustion chamber in an operation region on a low load side in the partial load region. Furthermore, in the engine, in an operation region on a high load side in the partial load region, cooled burned gas is introduced into the combustion chamber such that compression autoignition is less likely to occur, and a spark plug performs ignition immediately before the compression top dead center.